CN114867879B

CN114867879B - Ferritic stainless steel material and method for producing same

Info

Publication number: CN114867879B
Application number: CN202180007384.8A
Authority: CN
Inventors: 弘中明; 山本大智
Original assignee: Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2020-04-15
Filing date: 2021-04-08
Publication date: 2023-07-07
Anticipated expiration: 2041-04-08
Also published as: TW202142709A; JP7278476B2; JPWO2021210491A1; KR20220092979A; TWI750077B; WO2021210491A1; CN114867879A

Abstract

The ferritic stainless steel (1) contains 1.0 mass% or more and 15.0 mass% or less of Cu, and is provided with: a base material (20) that forms a fine Cu-rich phase (21) having a particle diameter of less than 500nm in a matrix (25); and (ii) a Cu concentrated surface layer portion (10) formed on the surface of the ferritic stainless steel material (1) and having Cu more concentrated than the base material (20).

Description

Ferritic stainless steel material and method for producing same

Technical Field

The present invention relates to a ferritic stainless steel material and a method for producing the same.

Background

Conventionally, various techniques for improving the conductivity of stainless steel materials have been proposed in order to apply stainless steel materials to conductive members (e.g., electrical contact members) and the like.

For example, there is known a technique of forming a layer made of Ni or a Ni alloy on the surface of a stainless steel (patent document 1) or modifying the surface of a stainless steel (patent document 2) to reduce the surface contact resistance of the stainless steel.

Patent document 2 describes a technique for modifying the surface of a stainless steel material. The following are specifically described: concentrating Cu on the passivation film or the outermost layer of the stainless steel, and (ii) precipitating a second phase mainly composed of Cu on the surface of the stainless steel, thereby partially preventing the formation of the passivation film.

Patent document 3, for example, describes a stainless steel material in which Cu-rich phases are aged and precipitated in a ferrite phase matrix to reduce the base material resistance.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open publication No. 2013-087329

Patent document 2: japanese patent laid-open No. 2001-089865

Patent document 3: japanese patent laid-open No. 2004-277807

Disclosure of Invention

Problems to be solved by the invention

However, in the technique described in patent document 1, it is necessary to ensure that the base material of the stainless steel is austenitic stainless steel and to form a Ni-containing layer on the surface of the base material, and therefore, it is difficult to reduce the manufacturing cost. Also, stainless steel generally has the following tendency: the higher the content of the alloy component (high alloy), the higher the electrical resistance of the base material.

In the technique described in patent document 2, although the surface contact resistance of the stainless steel material can be reduced, there is room for improvement in reducing the base material resistance in the stainless steel material.

In the technique described in patent document 3, although the base material resistance in the stainless steel material can be reduced, there is room for improvement in reducing the surface contact resistance of the stainless steel material.

An embodiment of the present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a ferritic stainless steel material having both reduced electric resistance and surface contact resistance, and a method for producing the same.

Means for solving the problems

In order to solve the above problems, a ferritic stainless steel according to an embodiment of the present invention is a ferritic stainless steel containing 1.0 mass% or more and 15.0 mass% or less of Cu, comprising: a base material that forms a first Cu-rich phase having a particle diameter of less than 500nm in a matrix; and a Cu concentrated surface layer portion formed on the surface of the ferritic stainless steel material, wherein Cu is more concentrated than the base material.

The method for producing a ferritic stainless steel according to an embodiment of the present invention includes: an annealing step of heating a rolled material composed of a ferritic stainless steel component containing 1.0 mass% or more and 15.0 mass% or less of Cu at a temperature of 780 ℃ or more and 830 ℃ or less for 6 hours or more; an aging treatment step of forming a first Cu-rich phase having a grain size of less than 500nm in a base by aging under a condition that an A value defined by the following formula (1) is 15.0 or more and 20.0 or less, with respect to a first intermediate material obtained by performing an intermediate step including at least a first pickling step, after the annealing step; and a second pickling step of performing a pickling treatment on the second intermediate material obtained in the aging treatment step by using a mixed solution containing (i) nitric acid having a concentration of 50g/L or more and 150g/L or less and (ii) hydrofluoric acid having a concentration of 5g/L or more and 15g/L or less, the mixed solution having a liquid temperature of 30 ℃ or more and 60 ℃ or less;

A＝T(20+logt)×10 ^-3 ···(1)

(wherein,

t: aging temperature (K)

t: aging time (h)).

Effects of the invention

According to an embodiment of the present invention, a ferritic stainless steel material having both reduced electric resistance and reduced surface contact resistance and a method for producing the same can be provided.

Drawings

Fig. 1 is a cross-sectional view schematically showing a ferritic stainless steel material according to an embodiment of the present invention.

Fig. 2 is a graph showing an example of Cu strength change in the depth direction in the vicinity of the surface of a ferritic stainless steel.

Detailed Description

An embodiment of the present invention will be described below. The following description is not intended to limit the present invention unless otherwise specified, in order to better understand the gist of the present invention. In the present specification, "a to B" means a or more and B or less.

(definition of terms)

The "ferritic stainless steel" may be a steel strip, a steel sheet, or the like, and the specific shape of the steel is not limited. In this embodiment, a ferritic stainless steel strip will be described as an example of a ferritic stainless steel material. In addition, since the term "steel sheet" may be regarded as a part of the term "steel strip", the term "ferritic stainless steel sheet" is used in a sense including "ferritic stainless steel strip".

The "surface contact resistance value" is an index indicating the contact resistance of the surface of the stainless steel material, and generally indicates that the surface contact resistance value of the stainless steel material is a high value. This is because a passivation film exists on the surface of the stainless steel.

The "resistivity" is an index indicating how easily current flows through the steel (i.e., the base material) of the stainless steel material. Stainless steel is a high alloy and therefore generally exhibits relatively high resistivity.

"scratch resistance" refers to a property related to the degree of difficulty in producing scratches on the surface of stainless steel. It can be said that the harder the surface of the stainless steel material is, the higher the scratch resistance is.

The "Cu-rich phase" refers to a second phase mainly composed of Cu, which is formed in the structure of a Cu-containing stainless steel material, and is a phase containing 80 atomic% or more of Cu.

(about conventional methods of preparation)

First, an example of a conventional stainless steel strip manufacturing process will be briefly described. The conventional stainless steel strip manufacturing process includes, as an example, a steel-making process, a hot rolling process, an annealing process, an acid pickling process, a cold rolling process, an annealing/acid pickling process, and a finish rolling process in this order. Since each of these conventional production steps is well known, a detailed description thereof is omitted except for the following description.

Each step after the hot rolling step is usually performed using a coil of steel formed from a coiled stainless steel strip. That is, the treatment is continuously performed on the stainless steel strip guided out from the coil, and the treated stainless steel strip is rewound into the coil.

In recent years, in the annealing step, from the viewpoint of cost reduction, it is often necessary to continuously anneal a stainless steel strip, and in this case, it is necessary to continuously perform an annealing treatment on a stainless steel strip led out from a steel coil by an annealing line. In this regard, there is also a method of directly annealing the steel coil in a heating furnace (e.g., a hood-type annealing furnace) for a relatively long period of time, and such a method is called batch annealing (also called box annealing or hood-type annealing).

When pickling an annealed stainless steel strip, various methods are used to pickle scale (scale) on the surface of the stainless steel strip. For example, as the pickling solution used for pickling, a mixed solution of nitric acid and hydrofluoric acid, an acid solution containing sulfuric acid, or the like can be used. In addition, the annealed stainless steel strip may be electrolytically pickled using, for example, nitric acid.

(summary of invention knowledge)

In the case of applying the stainless steel to a conductive member or the like, it is necessary to reduce the surface contact resistance of the stainless steel and the resistance of the base material thereof.

In addition, the application of stainless steel to an electrical contact member which is one of conductive members has been studied. For example, in a component such as a terminal, when connecting to another circuit, an operation of inserting the component into a connector is performed. The surface of the component is thus scratched. Such surface scratches may reduce the stability of the component.

Therefore, as a stainless steel material for electric contact members, it is required to reduce both the electric resistance and the surface contact resistance and to improve the scratch resistance. In addition, as a stainless steel material for electric contact members, it is also required that the manufacturing cost is not increased.

The present inventors have studied about ferrite stainless steel suitable for use as an electrical contact member among ferrite stainless steel having a low cost. The following knowledge was obtained as a result, and the invention of the present application was devised.

That is, by performing batch annealing for a long period of time on a steel material having a composition of a ferrite stainless steel containing Cu, a large amount of Cu-rich phase having a relatively large grain size can be precipitated in a base material. Then, an intermediate material (first intermediate material) obtained by performing an intermediate treatment after batch annealing is subjected to an aging treatment, whereby a Cu-rich phase is further precipitated in the base material. By defining the conditions for the aging treatment, a fine Cu-rich phase (first Cu-rich phase) having a smaller and finer grain size than the Cu-rich phase (second Cu-rich phase) produced by the batch annealing can be precipitated in the base material.

Further, among a plurality of pickling treatments which may be included in the ferritic stainless steel production process, at least the pickling treatment conditions after the aging treatment are defined. The following phenomenon occurs in the pickling treatment after the aging treatment. That is, cu ions eluted from the surface of the intermediate material (second intermediate material) after the aging treatment and contained in the pickling solution are reattached to the surface of the intermediate material (ferritic stainless steel) in the pickling treatment. Here, by performing the batch annealing and the time-efficient treatment, the base material of the intermediate material includes a Cu-rich phase and a fine Cu-rich phase. Therefore, the Cu-rich phase and the fine Cu-rich phase existing in the vicinity of the surface of the intermediate material dissolve, and the concentration of Cu ions eluted in the pickling solution can be increased. As a result, a Cu concentrated portion can be formed appropriately on the surface of the ferrite stainless steel material and the vicinity thereof. In the present specification, the surface of the ferritic stainless steel material in which the Cu concentrated portion is formed and the vicinity thereof are referred to as Cu concentrated surface layer portions. The Cu concentrated surface layer portion includes a Cu adhesion layer formed by precipitating Cu ions in the pickling solution, a Cu-rich phase, and a fine Cu-rich phase. Thereby, the surface contact resistance can be reduced.

The fine Cu-rich phase is a phase mainly composed of Cu and formed in a structure of a Cu-containing stainless material, and is a phase containing 80 atomic% or more of Cu, similarly to the Cu-rich phase.

By these effects, a ferritic stainless steel material having both of an improved surface hardness and a reduced electric resistance and surface contact resistance is realized. Further, since the ferritic stainless steel according to the embodiment of the present invention includes a fine Cu-rich phase in the Cu concentrated surface layer portion, the surface hardness is improved and the scratch resistance is also improved. By using such a ferritic stainless steel material, an electrical contact member having stable electrical conductivity can be produced.

< ferritic stainless Steel material >)

The ferrite stainless steel according to an embodiment of the present invention will be described below with reference to fig. 1. Fig. 1 is a cross-sectional view schematically showing a ferritic stainless steel material according to an embodiment of the present invention.

As shown in fig. 1, the ferritic stainless steel 1 of the present embodiment has a fine Cu-rich phase (first Cu-rich phase) 21 having a grain size of less than 500nm and a Cu-rich phase (second Cu-rich phase) 22 having a grain size of 500nm or more formed in a matrix 25. The ferritic stainless steel 1 according to the present embodiment has a Cu concentrated surface layer portion 10 in which Cu is more concentrated than the matrix 25, formed on the surface of the base material 20 and in the vicinity thereof. The matrix 25 is a main phase (parent phase) in the base material 20, and is a substantially ferrite single-phase structure. The parent phase in the parent material 20 is a ferrite single-phase structure, and the portion (base material) of the metal structure of the parent material 20 excluding the Cu-rich phase is a structure substantially composed of ferrite phase. By "substantially" is meant that other phases (e.g., precipitates or inclusions) are allowed to mix within a range of about 3% by volume or less.

Further, the Cu concentrated surface layer portion 10 includes a passivation film 11, a Cu adhesion layer 13, and a base surface layer 25A as a surface layer portion of the base 25. The ferritic stainless steel 1 has a fine Cu-rich phase 21 and a Cu-rich phase 22 formed in a base surface layer 25A. Each of these will be described in detail later.

In fig. 1, the structure of the Cu concentrated surface layer 10 and the base material 20 is schematically shown, and the shape, size, and position of each portion included in the Cu concentrated surface layer 10 and the base material 20 are assumed for convenience of illustration, but the present invention is not limited thereto.

(composition of ingredients)

The ferritic stainless steel 1 contains Cu:1.0 mass% or more and 15.0 mass% or less. The ferritic stainless steel 1 is based on the composition of ferritic stainless steel and contains Cu. That is, the Ni content in the ferritic stainless steel 1 is 1.0 mass% or less.

Cu is added to improve the conductivity of the ferritic stainless steel 1. If the Cu content is less than 1.0 mass%, the conductivity cannot be sufficiently improved by the treatment described later. On the other hand, if the Cu content is too large, there is a possibility that the hot workability and corrosion resistance are lowered, and thus the Cu content is limited to 15.0 mass% or less. In the case of manufacturing a steel sheet or the like, if the cost is significantly increased due to deterioration of hot workability, cu is preferably contained in a range of 1.0 mass% or more and 8.0 mass% or less. Further, by containing a large amount of Cu (for example, containing Cu more than 5 mass%), the ferritic stainless steel 1 is more likely to further lower both the surface contact resistance value and the electrical resistivity. Further, by containing a large amount of Cu, the density of the fine Cu-rich phase 21 can be made relatively high, and as a result, the surface hardness can be easily increased.

Cr is an essential element for improving corrosion resistance of steel, and the ferritic stainless steel 1 preferably contains Cr:9.0 mass% or more and 20.0 mass% or less. If Cr is excessively added, the conductivity is lowered and the manufacturability is deteriorated, so that the Cr content is limited to 20.0 mass% or less.

In view of the balance between hot workability and electric resistance, the ferritic stainless steel 1 preferably contains Cu:1.5 mass% or more, 5.0 mass% or less, cr:11.0 mass% or more and 13.5 mass% or less.

The mass% of the alloying elements other than Cr and Cu is c+n: less than 0.10%, mn: below 2.0%, si: less than 2.0%, and if necessary, ti: less than 0.5%, nb: 1 or 2 of 0.5% or less, and the balance may be Fe and unavoidable impurities. In addition, if necessary, the composition may be represented by mass% in Mo:3.0% or less, ni: less than 3.0%, al: less than 5.0%, V: below 2.0%, W: less than 2.0%, zr: less than 1.0%, REM: 1 or 2 or more of these elements are contained in a range of 0.1% or less.

Contains Ti: less than 0.5%, nb: at 0.5% or less of 1 or 2, the following formula (2) is preferably satisfied:

7(C+N)≤Ti+Nb≤7(C+N)+0.3···(2)。

(Cu-rich phase/micro Cu-rich phase)

The fine Cu-rich phase 21 is an aged precipitate dispersed and precipitated in the matrix 25 by an aged precipitation treatment described later. The particle size of the fine Cu-rich phase 21 is more than 0 and less than 500nm. The lower limit of the particle diameter is not particularly limited as long as it is a particle diameter that can be observed by a transmission electron microscope. The particle size of the fine Cu-rich phase 21 is preferably 5nm to 20 nm. The particle diameter of each particle of the fine Cu-rich phase 21 can be expressed by the maximum diameter of the particle. Although it is difficult to quantitatively measure the particle size of each ultrafine particle, it is completely possible to determine whether the particle size of the heterogeneous phase dispersed in the matrix 25 is within a range of less than 500nm by observation with a transmission electron microscope. Further, it can be determined whether or not the particle diameter of the fine Cu-rich phase 21 is in the range of 5nm to 20 nm.

The Cu-rich phase 22 is a precipitate dispersed and precipitated in the matrix 25 by a batch annealing treatment described later. The Cu-rich phase 22 has a particle size of 500nm or more, preferably 1500nm or more.

The ferritic stainless steel according to an embodiment of the present invention includes at least the fine Cu-rich phase 21 in the matrix 25. Thereby, the resistivity of the base material 20 can be reduced. The ferritic stainless steel 1 according to the present embodiment has a microstructure in which the fine Cu-rich phase 21 and the Cu-rich phase 22 coexist in the matrix 25, and can further effectively reduce the electrical resistivity of the base material 20. In particular, the effect of improving conductivity is more remarkable by causing the fine Cu-rich phase 21 having a particle diameter of 5nm or more and 20nm or less and the Cu-rich phase 22 having a particle diameter of 1500nm or more to coexist and disperse in the matrix 25 as the ferrite phase.

The reason for this is not clear, but for example, it is considered that the distance between the fine Cu-rich phase 21 and the Cu-rich phase 22 in the base material 20 becomes short, and a conductive path is formed.

Here, the fine Cu-rich phase 21 and the Cu-rich phase 22 are formed in the matrix 25 so as to have a particle size distribution. It is difficult to measure the particle size (maximum diameter of particles) of each fine particle (precipitated phase) and to quantitatively express the particle size distribution by some index for the fine Cu-rich phase 21 and the Cu-rich phase 22. On the other hand, the fine Cu-rich phase 21 and the Cu-rich phase 22 can exhibit an average particle diameter and a particle size distribution within which ranges. The particle size of each microparticle can be measured by, for example, observation with a transmission electron microscope.

Therefore, in the present specification, the fine Cu-rich phase 21 and the Cu-rich phase 22 have particle size ranges defined by the following rules R1 and R2.

Rule R1: comprising the range of average particle diameters of the particles measured.

Rule R2: the particle diameter of most (80%) of the particles to be measured falls within the range.

In the ferritic stainless steel material 1 of the present embodiment, the Cu-rich phase 22 may have a particle diameter of 500nm or more and 2500nm or less, may be 500nm or more and 2000nm or less, or may be 500nm or more and 1500nm or less.

Based on the above rules R1 and R2, the following conclusions can be drawn. That is, when the ferritic stainless steel 1 includes, for example, the Cu-rich phase 22 having a particle size of 500nm or more and 2500nm or less, fine particles (Cu-rich phase 22) having a particle size of 500nm or more and less than 2000nm are formed in the matrix 25.

The ferrite stainless steel 1 having the Cu-rich phase 22 is compared with a high Cr steel for an electric conduction member described in patent document 3, for example, and is described below.

The high Cr steel for a conductive member described in patent document 3 contains a Cu-rich phase (hereinafter, abbreviated as residual Cu-rich phase RCP for convenience of description) having a grain size of 2000nm or more, which remains undissolved before aging treatment. The residual Cu-rich phase RCP is a phase different from "aged precipitates" generated in the matrix 25 by aging.

The Cu-rich phase 22 included in the ferritic stainless steel 1 of the present embodiment is a precipitate dispersed and precipitated in the matrix 25 by a batch annealing treatment described later, and is different from the residual Cu-rich phase RCP. In the ferritic stainless steel 1 of the present embodiment, a large number of Cu-rich phases 22 having a small particle size can be dispersed in the matrix 25, as compared with the high Cr steel for the current carrying member described in patent document 3.

The ferritic stainless steel material 1 according to the present embodiment can reduce the electrical resistivity of the base material as compared with the high Cr steel material for the current carrying member described in patent document 3. The distance between the Cu-rich phases (fine Cu-rich phase 21 and Cu-rich phase 22) in the ferritic stainless steel 1 is relatively shortened, and as a result, it is considered that a conductive path contributing to improvement of resistivity is effectively formed.

(Cu concentrated surface layer portion)

As shown in fig. 1, a Cu concentrated surface layer portion 10 in which Cu is more concentrated than the matrix 25 is formed on the surface of the ferritic stainless steel material 1. The Cu concentrated surface layer portion 10 is formed by performing a pickling process (pickling process using a mixed acid) described later after the base material 20 is brought into a state of at least including the Cu-rich phase 22 by a batch annealing process and a time-lapse process described later. Hereinafter, the pickling process for forming the Cu concentrated surface layer portion 10 will be referred to as Cu adhesion pickling process in this specification. The Cu adhesion pickling treatment is performed at least as a final pickling treatment in the manufacturing process of the ferritic stainless steel material 1. The Cu adhesion pickling treatment is preferably performed in an intermediate step after batch annealing and before the final pickling treatment. The reason for this is as follows.

The Cu concentrated surface layer 10 will be described with reference to fig. 2. Fig. 2 is a graph showing an example of Cu strength change in the depth direction, which is detected by glow discharge spectral surface analysis of the surface of the ferritic stainless steel material 1. Specifically, the Cu distribution in the depth direction was measured from the surface of the ferritic stainless steel material 1 by Glow Discharge Spectroscopy (GDS) using a test piece having a width of 50mm and a length of 50 mm. Here, it can be said that the Cu concentration of the base surface layer 25A in which the fine Cu-rich phase 21 and the Cu-rich phase 22 are formed is the same as the Cu concentration of the base material 20. Therefore, as the Cu concentration of the base material 20 (represented by the base material strength in fig. 2), for example, an average value from the surface of the ferritic stainless steel material 1 to a depth of 40nm to 50nm is used.

As shown in fig. 2, when the ferritic stainless steel material 1 of the present embodiment is subjected to composition analysis, the peak intensity (surface layer intensity) I of the Cu concentration at the surface and the vicinity thereof is significantly larger than the intensity (base material intensity) IO of the Cu concentration of the base material 20. Such a surface layer portion is referred to as a Cu concentrated surface layer portion 10. The Cu concentrated surface layer portion 10 has a Cu concentrated portion, i.e., a portion where the surface layer strength I of the Cu concentration is obtained, i.e., the Cu adhesion layer 13. The Cu adhesion layer 13 is formed in a region from the surface of the ferritic stainless steel material 1 to a depth of about 10 nm. The surface layer strength I means a peak strength in a graph showing the Cu concentration of the Cu concentrated surface layer section 10. In the example shown in fig. 2, the skin strength I is about 0.6.

In the ferritic stainless steel material 1 of the present embodiment, the ratio of the surface strength I of the Cu concentrated surface layer portion 10 (more specifically, the Cu adhesion layer 13) to the strength IO of the Cu concentration of the base material 20 (surface Cu strength ratio) is 1.5 or more, for example, 1.6 or more and 2.5 or less. The surface Cu strength ratio of the ferritic stainless steel material 1 is preferably 1.7 or more in view of stability of the surface contact resistance value. The surface Cu strength ratio of the ferritic stainless steel 1 is preferably 2.0 or less. This is because if the surface Cu strength ratio exceeds 2.0, there is a possibility that the corrosion resistance of the ferritic stainless steel material 1 may be lowered. The surface layer Cu strength ratio of the ferritic stainless steel material 1 may be 1.5 or more and 2.0 or less, and preferably 1.7 or more and 2.0 or less.

The detailed structure of the Cu concentrated surface layer portion 10 is not known so far, but as described above, the surface layer Cu strength ratio is 1.5 or more, and therefore, it is considered to have a structure including the Cu adhesion layer 13, the passivation film 11, and the base surface layer 25A. The surface layer 25A is a portion near the outermost surface of the matrix 25 (for example, a portion from the outermost surface to a depth of several μm), and includes a fine Cu-rich phase 21 and a Cu-rich phase 22 (see fig. 1). The base surface layer 25A is a part of the base 25.

The Cu concentrated surface layer portion 10 includes a Cu adhesion layer 13, and the Cu adhesion layer 13 is formed by adhering (reattaching) Cu ions eluted in the pickling solution in the Cu adhesion pickling treatment to the surface of the base material 20. The Cu adhesion layer 13 is a phase containing 80 atomic% or more of Cu, and may contain oxidized or oxidized Cu. The thickness of the Cu adhesion layer 13 is, for example, 2nm to 20nm. The Cu adhesion layer 13 is formed by adhering Cu ions to the surface of the base material 20 in a spatially sparse state (for example, in a porous state). Therefore, the Cu adhesion layer 13 is formed so as to have communication holes that communicate the atmosphere and the base surface layer 25A with each other. The communication holes are formed so that at least oxygen can move from the atmosphere to the surface layer 25A of the substrate.

The Cu concentrated surface layer portion 10 of the ferritic stainless steel material 1 of the present embodiment has a passivation film 11, and the passivation film 11 is formed in a lower layer (inner side) deeper than the Cu adhesion layer 13 in the atmosphere.

The passivation film 11 is formed by reacting (i) oxygen gas, which contacts the base surface layer 25A, of the porous Cu adhesion layer 13 (specifically, the communication holes) with (ii) Cr or the like contained in the base surface layer 25A, so as to be formed in at least a part of the interface between the Cu adhesion layer 13 and the base surface layer 25A. The porous Cu adhesion layer 13 includes irregularly shaped pores (air holes), and a portion may have an open pore shape in which the space within the pores communicates.

More specifically, in the Cu adhesion pickling treatment, the passivation film existing before the treatment is broken, and the Cu adhesion layer 13 is formed on the surface of the substrate 25 (or the substrate surface layer 25A). After the Cu adhesion pickling treatment described above, oxygen is supplied in the atmosphere through the Cu adhesion layer 13 in a porous state, so that the passivation film 11 is formed in at least a part of the interface between the base surface layer 25A and the Cu adhesion layer 13.

The base surface layer 25A of the Cu concentrated surface layer section 10 includes a fine Cu-rich phase 21 and a Cu-rich phase 22.

As shown in fig. 1, the surface of the base 25 (i.e., the interface between the Cu adhesion layer 13 and the base surface layer 25A) has a region where the fine Cu-rich phase 21 or the Cu-rich phase 22 is present, and the passivation film 11 may not be formed in the Cu-concentrated surface layer portion 10. This is because the amount of Cr present in this region is insufficient, and it is difficult to form the passivation film 11. In this case, the Cu adhesion layer 13 and the fine Cu-rich phase 21 or the Cu-rich phase 22 may be in contact with each other, and thus a good conductive region (conductive path) having relatively high conductivity may be formed.

In addition, in the region where the oxygen supply through the porous Cu adhesion layer 13 is insufficient, the Cu concentrated surface layer section 10 can bring the Cu adhesion layer 13 and the base surface layer 25A into contact with each other, and also form a good conductive region having relatively high conductivity.

The Cu concentrated surface layer portion 10 includes a Cu adhesion layer 13 and a base surface layer 25A on the surface, and includes a passivation film 11 at least in a part of the interface between the Cu adhesion layer 13 and the base surface layer 25A. The Cu concentrated surface layer portion 10 is formed by bringing the Cu adhesion layer 13 and the fine Cu-rich phase 21 or the Cu-rich phase 22 into contact with each other, thereby forming a conductive path having relatively high conductivity. Thus, the ferritic stainless steel 1 can lower the surface contact resistance more than the general ferritic stainless steel.

In the production process, the ferritic stainless steel material 1 of the present embodiment is not subjected to the polishing treatment before the Cu adhesion pickling treatment. Therefore, the ferritic stainless steel 1 does not have polishing marks on the surface of the substrate 25 or the substrate surface layer 25A. As a result, the surface of the ferritic stainless steel material 1 also has no grinding marks. The surface roughness (arithmetic average roughness Ra) of the ferritic stainless steel material 1 may be, for example, 0.5 μm or less, or may be 0.01 μm or more and 0.5 μm or less.

(conductivity)

The ferrite stainless steel 1 of the present embodiment has a resistivity of 60 μΩ·cm or less and a surface contact resistance value of 45mΩ or less. The ferrite stainless steel 1 preferably has a resistivity of 60 μΩ·cm or less and a surface contact resistance value of 30mΩ or less.

The resistivity of the ferritic stainless steel material 1 may be 50 μΩ·cm or less, or may be 40 μΩ·cm or less. The surface resistance value of the ferritic stainless steel material 1 may be 20mΩ or less, or may be 10mΩ or less. The ferrite stainless steel 1 may have a surface contact resistance value of 20mΩ or less while the resistivity is reduced to 50 μΩ·cm or may have a surface contact resistance value of 10mΩ or less while the resistivity is reduced to 50 μΩ·cm or less.

The resistivity and the surface contact resistance can be measured by the methods described in examples described below.

< manufacturing method >)

An example of the method for producing a ferritic stainless steel according to the present embodiment will be described below.

(pretreatment step)

In the pretreatment step, first, steel having a composition adjusted to be within the scope of the present invention is melted using a vacuum melting furnace. The steel is cast to produce a steel block.

(Hot Rolling Process)

In the hot rolling step, the steel block after the pretreatment step is hot-rolled to produce a hot-rolled steel strip. The temperature in the hot rolling step may be in a conventional range, for example, about 800 to 1250 ℃. In the hot rolling step, the hot rolled steel strip may be produced at a temperature of 1150 to 1250 ℃ for 30 to 120 minutes. Thereby making Cu more soluble in the texture of the hot rolled strip. In the hot rolling step, when the Cr content of the component composition is, for example, 9.0 mass% or more and 16.5 mass% or less, the hot rolled steel strip may be produced at a temperature of 1100 to 1180 ℃ for 30 to 120 minutes.

(first annealing step)

The method for producing a ferritic stainless steel sheet according to the present embodiment includes an annealing step of annealing (batch annealing) the hot-rolled steel strip having the ferritic stainless steel composition using, for example, a batch annealing furnace (hood-type annealing furnace). This annealing step is referred to as a first annealing step (batch annealing step). The heating temperature in the first annealing step is 780 ℃ or higher and 830 ℃ or lower, and the heating time is 6 hours or longer. In the first annealing step, the hot rolled steel strip is kept at the heating temperature for the heating time. In the first annealing step, the annealing may be performed in an atmospheric environment or N ₂ And H is ₂ The annealing is performed in any one of the mixed gas environments.

By performing the first annealing step on the hot-rolled steel strip, a large amount of Cu-rich phase 22 can be precipitated in matrix 25. In the first annealing step, the hot-rolled steel strip is softened by setting the heating temperature to 780 ℃ or higher.

In the first annealing step, the heating temperature is set to 830 ℃. The matrix 25 of the hot rolled steel strip may have a temperature range (i.e. the gamma region in the state diagram) that will transform into the austenitic phase due to its steel composition. If the matrix 25 is transformed into an austenite phase, the Cu-rich phase 22 is not precipitated in the matrix 25.

Therefore, the heating temperature in the first annealing step is set to a relatively narrow range of 780 ℃ or higher and 830 ℃ or lower.

In the first annealing step, the hot-rolled steel strip may be subjected to batch annealing under predetermined conditions using the a value defined by the following formula (1), as in the aging step described later.

A＝T(20+logt)×10 ^-3 ···(1)

Wherein T is a heating temperature (K) in the first annealing step, and T is a heating time (h) in the first annealing step.

The first annealing step may be performed under conditions in which the heating temperature is 780 ℃ or higher and 830 ℃ or lower, and the a value is larger than that of the aging step described later, and for example, the first annealing step may be performed under conditions (heating temperature and heating time) in which the a value is more than 20.0 and 24.0 or lower. If the first annealing step is performed under the condition that the a value is 20.0 or less, the Cu-rich phase 22 is not formed in a sufficient size, and it is difficult to sufficiently improve the conductivity of the base material. On the other hand, if the first annealing step is performed under the condition that the a value exceeds 24.0, the Cu-rich phase 22 becomes too coarse, and the distribution of the Cu-rich phase 22 becomes sparse. In this case, (i) formation of the conductive path may be insufficient, and (ii) the coarse Cu-rich phase 22 may become a fracture origin, and thus sufficient workability may not be obtained.

(intermediate step)

In the method for producing the ferritic stainless steel material 1 according to the present embodiment, an intermediate step including at least a first pickling step is performed after the first annealing step.

The annealed steel strip (first annealed material) obtained in the first annealing step is subjected to an acid cleaning treatment in the first acid cleaning step. In the first pickling step, the annealed steel strip is subjected to descaling (descaling) treatment.

In the first pickling step, cu adhesion pickling is preferably performed using a mixed acid under the same conditions as in the final pickling step described later. Specifically, it is preferable to use a pickling solution (mixed acid) containing (i) nitric acid in an amount of 50g/L to 150g/L and (ii) hydrofluoric acid in an amount of 5g/L to 15g/L, and to perform a pickling treatment on the annealed steel strip while the solution temperature of the pickling solution is in an amount of 30 ℃ to 60 ℃.

In the method for producing the ferritic stainless steel material 1 according to the present embodiment, the case where the Cu adhesion pickling treatment is performed in the first pickling step under the same conditions as in the final pickling step described later will be described.

By performing the Cu adhesion pickling treatment in the first pickling step, the surface of the annealed steel strip is removed of scale, and at the same time, a part of the matrix 25 (including the Cu-rich phase 22) is dissolved, whereby Cu ions are eluted in the pickling solution. In the first pickling step, cu ions contained in the pickling solution are reattached to the surface of the annealed steel strip. Thereby, a Cu adhesion layer is formed on the surface of the annealed steel strip. Hereinafter, the Cu adhesion layer formed on the surface of the material to be treated during the manufacturing process of the ferritic stainless steel 1 is referred to as an intermediate Cu adhesion layer so as to be distinguishable from the Cu adhesion layer 13 in the ferritic stainless steel 1. The intermediate Cu adhesion layer may have the same composition as the Cu adhesion layer 13. The total amount of Cu contained in the intermediate Cu adhesion layer is also relatively small compared to the total amount of Cu contained in the Cu adhesion layer 13.

The intermediate step may further include a cold rolling step after the first pickling step. When the intermediate step includes a cold rolling step, the annealed steel strip after the oxide scale is removed in the first pickling step is cold-rolled at a reduction ratio of, for example, 50% to 80%, to produce a cold-rolled steel strip. By making the annealed steel strip thin, the ferritic stainless steel material 1 suitable for use in electric contact members and the like can be suitably produced.

The intermediate step may further include a second annealing step and a second pickling step of annealing and pickling the cold-rolled steel strip after the cold-rolling step.

The second annealing step may be continuous annealing, and the treatment time may be, for example, about several tens of seconds to several minutes. The soaking temperature in the second annealing step may be about 800 ℃. The second annealing step is performed to soften the cold-rolled steel strip. In the second annealing step, the heated steel strip is air-cooled to form a second annealed steel strip.

In the second annealing step, supersaturated Cu is preferably secured in the matrix 25 so as to perform an aging step described later. Therefore, by performing the second annealing step under the following temperature conditions, the Cu-rich phase 22 precipitated in the first annealing step can be slightly dissolved, and in this case, the grain size of the Cu-rich phase 22 is slightly reduced.

The second annealing step may be performed at a temperature of 780 ℃ or higher. This is to ensure a sufficient temperature for softening the material. On the other hand, the second annealing step is performed at a temperature of 850 ℃. This is because, if the temperature exceeds 850 ℃, the re-solid solution of the Cu-rich phase 22 becomes severe.

The second annealing step is performed at a temperature of 850 ℃ or lower and less than the Ac1 point. The Ac1 point can be calculated by the following formula.

Ac1＝750.8－26.6C+17.6Si－11.6Mn－22.9Cu－23Ni+24.1Cr+22.5Mo－39.7V－5.7Ti+232.4Nb－169.4Al－894.7B

Here, in the above element symbol, mass% in the component composition may be substituted.

In the second annealing step, the cold-rolled steel strip is heated at a desired temperature and time and then rapidly cooled. This reduces the possibility of re-precipitation of Cu in the matrix 25 during cooling after heating. As a result, the second annealed steel strip having the matrix 25 in which supersaturated Cu is ensured can be obtained.

Next, a second pickling step is performed on the second annealed steel strip after the second annealing step. And performing descaling treatment of the second annealed steel strip by using the second pickling step. In the second pickling step, the Cu adhesion pickling treatment is preferably performed under the same conditions as in the final pickling step described later. Specifically, it is preferable to use a pickling solution containing (i) nitric acid in an amount of 50g/L to 150g/L and hydrofluoric acid in an amount of 5g/L to 15g/L, and to perform the pickling treatment on the second annealed steel strip while the solution temperature of the pickling solution is in an amount of 30 ℃ to 60 ℃.

In the second pickling step, the surface scale of the second annealed steel strip is removed, and at the same time, the intermediate Cu adhesion layer, the passivation film 11, and a part of the matrix 25 (including the Cu-rich phase 22) are dissolved. Here, the concentration of Cu ions in the pickling solution can be increased by dissolving the intermediate Cu adhesion layer and a part of the base surface layer 25A on the surface of the second annealed steel strip.

Thus, the annealed steel strip after the second pickling step (steel strip of intermediate product after intermediate step) can effectively form an intermediate Cu adhesion layer on the surface layer.

(aging treatment step)

The method for producing the ferritic stainless steel material 1 according to the present embodiment includes an aging step of aging a steel strip of an intermediate product (first intermediate material) after the intermediate step. The aging step is performed under conditions such that the A value defined by the following formula (1) is 15.0 to 20.0.

A＝T(20+logt)×10 ^-3 ···(1)

Wherein T is the ageing temperature (K), and T is the ageing time (h).

By performing the aging treatment step as described above, the fine Cu-rich phase 21 can be dispersed and precipitated in the matrix 25 of the steel strip of the intermediate product.

In the aging step, it is assumed that the aging time t is preferably 0.016h or longer (estimated to be about 60sec or longer) as the on-line treatment. In addition, the aging temperature T is preferably 830 ℃ or lower so that no phase transition occurs in the matrix 25 (see the description of the first annealing step described above). The aging temperature T may be 500 to 830 ℃, or 500 to 700 ℃.

In the method for producing the ferritic stainless steel material 1 according to the present embodiment, Δhv calculated by the following formula (2) is larger than 35 HV:

ΔHV＝HV2－HV1···(2)

where HV1 is the hardness of the intermediate product (first intermediate material) before the aging process, and HV2 is the hardness of the intermediate product (second intermediate material) after the aging process.

The ferritic stainless steel 1 includes the fine Cu-rich phase 21 in the matrix 25 and also includes the Cu concentrated surface layer portion 10. This allows manufacturing such that Δhv is also higher than 35 HV. As a result, the surface hardness can be improved. Thus, scratch resistance is improved. Further, the ferrite stainless steel 1 is preferably greater than 60HV, more preferably greater than 200 HV.

(final acid washing step)

The method for producing the ferritic stainless steel material 1 according to the present embodiment includes a final pickling step, and the intermediate product (second intermediate material) obtained through the aging step is subjected to a final pickling treatment. In the final pickling step, a pickling treatment (mixed acid treatment) is performed using a mixed acid. The pickling solution (mixed acid) used in the final pickling step contains: (i) nitric acid of 50g/L or more and 150g/L or less and (ii) hydrofluoric acid of 5g/L or more and 15g/L or less. The pickling solution has a liquid temperature of 30-60 ℃.

In the mixed acid treatment, cu ions in the dissolved scale or in the base material are preferentially precipitated (attached) to the surface than other ions. Therefore, the concentration of Cu was confirmed on the surface layer. By using the effect of the mixed acid treatment, it is possible to reduce the surface contact resistance value. On the other hand, in the nitric acid electrolysis, the surface is always dissolved by the electrolytic treatment, and thus the precipitation (adhesion) of dissolved ions cannot be confirmed.

As in the first pickling step and the second pickling step, the Cu adhesion layer 13 is formed on the surface of the treated material in the final pickling step. As a result, the ferritic stainless steel material 1 having the Cu concentrated surface layer portion 10 can be obtained.

(advantageous effects)

As described above, the ferritic stainless steel material 1 according to the present embodiment includes the fine Cu-rich phase 21 and the Cu-rich phase 22 in the base material 20, thereby reducing the electrical resistance of the base material 20. Further, by providing the Cu concentrated surface layer portion 10, the surface contact resistance is reduced. Further, as described above, the surface hardness can be improved. That is, the ferritic stainless steel 1 has both reduced electric resistance and surface contact resistance, and improved scratch resistance.

The ferritic stainless steel 1 has a Cu adhesion layer 13 formed as follows. That is, as described above, the mixed acid treatment is performed in the intermediate step, so that the intermediate Cu adhesion layer is formed on the surface of the first intermediate material. Then, in the subsequent aging treatment step, cu of the intermediate Cu adhesion layer is diffused into tempering (thin oxide scale) to form a Cu-rich tempering. Then, the Cu-rich tempering color was dissolved by the final acid washing step. Thus, more Cu can be attached to the surface of the base surface layer 25A by the final pickling step, and the Cu adhesion layer 13 can be formed.

Since the ferritic stainless steel 1 is a steel containing a ferrite component, the Ni content is low, for example, the Ni content is 1.0 mass% or less. And, the surface does not need a special coating. Therefore, the manufacturing cost can be kept from increasing.

Therefore, when the ferritic stainless steel 1 is applied to an electric contact member, for example, both the electric resistance and the surface contact resistance are reduced, and thus the electric conductivity is excellent. In addition, the scratch resistance is improved, and the possibility of deterioration in conductivity can be reduced, and as a result, the electrical contact member can be used stably. Thus, the ferritic stainless steel material 1 can be suitably used as an electrical contact member.

[ notes ]

The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in the above description are also included in the technical scope of the present invention.

Examples

Examples of the ferritic stainless steel material in one embodiment of the present invention will be described below, but the present invention is not limited to these examples.

In this example, a ferritic stainless steel of the steel grade shown in Table 1 was melted in a vacuum melting furnace, and hot-rolled sheet having a thickness of 3mm was produced by hot rolling (1200 ℃ C., 2 hours). Next, batch annealing was performed by heating in a heating furnace at 700 to 830℃for 6 hours. Then, the resultant was pickled, and cold-rolled sheets having a sheet thickness of 1.0mm were produced by cold rolling. Each cold-rolled sheet was annealed by soaking at 800 ℃ for 1 minute and air-cooling, and then subjected to pickling treatment. Thereafter, aging treatment is performed at various temperatures ranging from 350 to 900 ℃, and pickling treatment is performed using a mixed acid (the composition of the components is referred to in the above-described embodiment) or pickling treatment is performed by electrolysis of nitric acid. The aging conditions are shown in tables 2 and 3.

TABLE 1

In table 1, the numerical values are not shown, but the Ni content of each of the steel grades C1 to C10 is 1.0 mass% or less. In the "classification" in table 1, the steel grades containing 1.0 mass% or more and 15.0 mass% or less of Cu are referred to as "the object steel of the present invention", and the steel grades having a Cu content of less than 1.0 mass% are referred to as "the control steel".

The grain size of the Cu-rich phase dispersed in the ferrite matrix was examined by observation with a transmission electron microscope on the steel sheet after batch annealing and after time-lapse treatment. In order to distinguish between the Cu-rich phase precipitated after batch annealing and after aging, a steel sheet was produced separately without batch annealing, and the steel sheet was subjected to aging, and the grain size of the Cu-rich phase in the aging was observed.

Further, the surface contact resistance value, the resistivity and the Cu concentration of the surface layer of the steel sheet after the aging treatment were tested. The surface contact resistance was measured by an electric contact simulator using a test piece having a width of 50mm×a length of 50 mm. The surface contact resistance value was measured under a measurement condition of a contact load of 100gf using an "electric contact simulator" manufactured by Kawasaki refiner research, kabushiki Kaisha. The resistivity was measured by the four terminal method (JIS C2525) using a test piece having a width of 3mm and a length of 100 mm. The Cu distribution in the depth direction was measured from the surface of the surface layer according to GDS using a test piece 50mm wide by 50mm long. The surface layer strength ratio is obtained by dividing the surface layer strength I, which is the peak value of the Cu concentration in the obtained Cu concentrated surface layer portion 10, by the strength IO of the Cu concentration of the base material 20 (average value from the surface of the ferritic stainless steel material 1 to the portion 40nm to 50nm in depth).

The surface hardness of the steel sheet before aging treatment and the surface hardness of the steel sheet after aging treatment were measured, respectively, and the difference between these was calculated.

The results are shown in tables 2 and 3. Table 3 shows steel sheets having the following characteristics (a) and (B) as "examples of the present invention". Table 2 shows a steel sheet having no properties (a) or (B) as a "control".

Characteristics (a): the surface contact resistance value is 45mΩ or less.

Characteristics (B): the resistivity is 60 mu Ω & cm or less.

TABLE 2

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TABLE 3

As shown in table 2, in comparative examples nos. 2, 4, 7, and 10 in which nitric acid electrolysis was performed in the fine pickling treatment, the surface contact resistance value was high regardless of the presence or absence of the aging treatment. In comparative example No.4, although the fine Cu-rich phase 21 was formed, the surface contact resistance value was high. The reason for this is considered to be that the Cu concentrated surface layer section 10, particularly the Cu adhesion layer 13, is not sufficiently formed. In comparative example No.4, since the surface Cu strength ratio was 1.1 and less than 1.5 as a reference, it was estimated that the Cu concentrated surface layer portion 10 was not sufficiently formed.

Further, as is clear from the results of comparative example No.1, if steel grade C1 having a low Cu concentration is used, the surface contact resistance value and the resistivity cannot be reduced even by the manufacturing method of the above embodiment.

In comparative examples 3, 5, 6 and 8, the aging conditions were outside the range of the present invention, and the base material resistivity was high. Specifically, in comparative example No.5, since the aging treatment is performed at a high temperature for a long period of time, the fine Cu-rich phase dissolves, and Cu is dissolved in the matrix. In addition, in comparative examples 3 and 6, the aging treatment temperature was low, and precipitation of a fine Cu-rich phase hardly occurred. Therefore, the resistivities of the base materials of comparative examples nos. 3, 5, and 6 were high. In comparative example No.8, no aging treatment was performed, and little precipitation of a fine Cu-rich phase occurred. From these results, in comparative examples No.3, 5, 6 and 8, the base material had a resistivity higher than 60. Mu. Ω & cm, and the ΔHV indicating the surface hardness was also 35 or less.

In comparative example No.9, the temperature by batch annealing was low, resulting in a small grain size of the Cu-rich phase precipitated. Therefore, the amount of fine Cu-rich phase precipitated in the aging treatment is small. As a result, the base material has a resistivity higher than 60 μΩ·cm, and a Δhv indicating the surface hardness is also 35 or less.

In contrast, as shown in table 3, the steel sheets of examples No.11 to 29 produced under the conditions within the scope of the present invention were large in Δhv and excellent in scratch resistance while both the surface contact resistance value and the electrical resistivity were reduced.

Symbol description

1. Ferritic stainless steel material

10 Cu concentrated surface layer part

20. Base material

21. Fine Cu-rich phase

22. Cu-rich phase

25. Matrix body

Claims

1. A ferritic stainless steel material containing 1.0 mass% or more and 15.0 mass% or less of Cu,

the device is provided with:

a base material that forms a first Cu-rich phase having a particle diameter of less than 500nm in a matrix; and

a Cu concentrated surface layer portion formed on the surface of the ferritic stainless steel material, the Cu being more concentrated than the base material,

wherein the base material forms a second Cu-rich phase having a particle size of 500nm or more in the matrix.

2. The ferritic stainless steel according to claim 1, wherein a ratio of a Cu strength peak value of the Cu concentrated surface layer portion to a Cu strength value of the base material, which is detected by a Cu-related composition analysis, is 1.5 or more.

3. The ferritic stainless steel according to claim 1 or 2, wherein the first Cu-rich phase has a grain size of 5nm or more and 20nm or less.

4. The ferritic stainless steel according to claim 1 or 2, wherein the ferritic stainless steel contains 9.0 mass% or more and 20.0 mass% or less of Cr.

5. The ferritic stainless steel according to claim 3, wherein the steel contains 9.0 mass% or more and 20.0 mass% or less of Cr.

6. The ferritic stainless steel according to claim 1 or 2, wherein the electrical resistivity is 60 μΩ -cm or less.

7. The ferritic stainless steel according to claim 1 or 2, wherein a surface contact resistance value is 45mΩ or less.

8. A method for producing a ferritic stainless steel material, comprising:

an annealing step of heating a rolled material composed of a ferritic stainless steel component containing 1.0 mass% or more and 15.0 mass% or less of Cu at a temperature of 780 ℃ or more and 830 ℃ or less for 6 hours or more;

an aging treatment step of forming a first Cu-rich phase having a grain size of less than 500nm in a base by aging under a condition that an A value defined by the following formula (1) is 15.0 or more and 20.0 or less, with respect to a first intermediate material obtained by performing an intermediate step including at least a first pickling step, after the annealing step; and

a final pickling step of performing pickling treatment on the second intermediate material obtained in the aging treatment step by using a mixed solution containing (i) nitric acid having a concentration of 50g/L or more and 150g/L or less and (ii) hydrofluoric acid having a concentration of 5g/L or more and 15g/L or less, at a liquid temperature of 30 ℃ or more and 60 ℃ or less;

A＝T(20+logt)×10 ^-3 ···(1)

(wherein,

T: aging temperature (K)

t: aging time (h)).

9. The method for producing a ferritic stainless steel according to claim 8, wherein in the first pickling step, a mixed solution containing (i) 50g/L to 150g/L of nitric acid and (ii) 5g/L to 15g/L of hydrofluoric acid is used for the first annealed material obtained in the annealing step, and the pickling treatment is performed at a liquid temperature of 30 ℃ to 60 ℃.

10. The method for producing a ferritic stainless steel according to claim 8 or 9, wherein Δhv calculated by the following formula (2) is larger than 35 HV;

ΔHV＝HV2－HV1 ··· (2)

(wherein,

HV1: hardness of the first intermediate material

HV2: hardness of the second intermediate material).